Thermionic energy conversion is based on the direct emission, or "boiling off," of electrons from a heated metal surface (2700-3000.degree. F.), called the emitter. The electrons from this emitter traverse a small, cesium vapor-filled gap and pass into a colder metal surface (600-1000.degree. F.), called the collector. The collector is connected to an external load where electrical work is performed. The amount of energy available to perform electrical work, and the efficiency with which it can be performed, are increased substantially by incorporating controlled levels of oxygen into the converter's cesium atmosphere. Then a Cs.sub.2 O--Cs solution provides a source of cesium and oxygen. The invention confronts the challenging engineering problem of controlling and maintaining the dynamic equilibrium of liquid cesium-oxygen mixtures in the converter.